SOIL FERTILITY Nitrogen Application to Soybean at Early Reproductive Development

tion and nodulation, thus reducing N2 fixation capacity (Shibles, 1998). Application of N before planting or durNitrogen application during soybean [Glycine max (L.) Merrill] ing the early growth stages can suppress the N2 fixation reproductive stages has the potential to increase soybean productivity. The objective of this study was to determine the impact of N fertilizer process. Fertilizer N applied during soybean reproducapplied to the soil at the beginning pod growth stage on soybean yield tive stages (R1 to R5) might increase the capacity and and grain quality. Additional objectives were to study alternative N duration of the inorganic N utilization period while mainfertilizer and application practices that might enhance soybean use taining N2 fixation. of applied N. A field study was conducted at five locations in Iowa A study conducted in Kansas with irrigated soybean during 1999 and 2000. Nitrogen treatments were urea and polymerfound significant soybean yield response when N was coated urea broadcast and subsurface band placed between the rows applied between the R3 and full pod stage (R4) (Wesley at 45 and 90 kg N ha 1 and a no-N control. The study showed few, et al., 1998). Their work showed an average yield insmall, and inconsistent effects of N material, placement, and rate on crease of 464 kg ha 1 at six of eight sites. The two grain yield and quality components at individual sites or when comnonresponsive sites had low yield, below 3360 kg ha 1. bined across individual sites. There were no significant effects on grain yield, with only a 39 kg ha 1 increase from applied N. Grain protein, oil, Nitrogen application did not enhance grain protein and and fiber concentrations were the same with or without N application. oil. This study suggests that in soybean with high yield Aboveground plant dry matter (DM) at the R6 growth stage was potential (3700 kg ha 1 or greater), 22 kg N ha 1 would greater with the higher N rate, but plant DM with N application was produced a positive yield response. Gascho (1993) in lower than the no-N control. Nitrogen concentration in plant DM Georgia studied a variety of N application treatments was significantly increased with applied N. In conclusion, N application on irrigated soybean with high yield potential (3360– increased N concentration in R6 soybean plants, but N rate and alter3695 kg ha 1) at the R3 to R5 growth stages. Nitrogen native application practices had no positive effect on plant DM, grain increased yields by more than 340 kg ha 1 on sandy N concentration and removal, grain yield, or grain quality components. soils and up to 340 kg ha 1 on loamy soils over a period It was concluded that growers should not consider fertilizer N applied of 5 yr. Afza et al. (1987) conducted a study with N to soil during early reproductive stages as a method to increase soybean yield or grain quality. application during the R4 growth stage. The plants translocated 40 to 67% of applied N into the pods and resulted in a significant seed yield increase. Results also showed that lower N rates (less than 40 kg N ha 1) did not I in N fertilization of soybean has grown in inhibit N2 fixation. Studies showing positive response to the U.S. Midwest during recent years. Demand for N application have suggested that yield increases are high grain quality, recognition of large N requirement limited to irrigated systems with high yield potential. for seed fill, and new varieties with high yield potential Swine manure applied as an N source during soybean are renewing interest in this nontraditional management reproductive stages has also produced positive yield inpractice. Soybean uses biological N2 fixation to produce creases. Anderson (1998) applied fertilizer N at the R1 approximately half of its total N requirement (Harper, growth stage and liquid swine manure in three weekly to 1987). The remaining N is derived from soil inorganic 16-d intervals at R1 and middle reproductive stages. Grain N, mineralized organic matter, or residual N from the yield increases of 8% occurred with the manure treatprevious crop. Soil nitrate is the main N source utilized ments but not the fertilizer N application. Furthermore, up to the beginning pod growth stage (R3) (Pedersen, manure inhibited nodulation less than the fertilizer N 2004), with crop use depleting soil inorganic N. Nitrate treatment. An explanation proposed by Anderson (1998) utilization and reductase activity drops rapidly at this for this result was that the swine manure provided a slow time (Shibles, 1998). Soil nitrate availability is therefore continuous release of N during seed filling, and soybean related to the length of the inorganic N utilization peabsorbed manure ammonium before nitrification. riod. Significant N use does not occur from N2 fixation Schmitt et al. (2001) evaluated in-season N applicauntil approximately beginning bloom (R1), increases tions on soybean in Minnesota. Treatments of urea and in the late full-bloom stage (R2) and R3 stages, and polycoated urea applied broadcast and in subsurface continues slightly beyond the beginning seed stage (R5) bands increased soil nitrate (0to 30-cm depth) at the (Harper and Hageman, 1972). It has been shown that full seed stage (R6). Nitrogen did not increase grain high soil nitrate levels delay Bradyrhizobia spp. infecyields but slightly increased seed N removal and grain Dep. of Agron., Iowa State Univ., Ames, IA 50011-1010. Received protein. The overall conclusion was that N applied dur20 Aug. 2004. *Corresponding author ([email protected]). ing the soybean reproductive stages would not increase yield and was not a recommended practice. Research Published in Agron. J. 97:615–619 (2005). © American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: DM, dry matter.